Recovering data in a near field communication apparatus

ABSTRACT

A communication device is disclosed that includes an antenna circuit with coupling connections that are used interchangeably as a receive coupling connection and a transmit coupling connection for an RF signal. A driver maintains a constant voltage or a constant current on a first coupling connection of the antenna circuit based on a drive signal that includes an output voltage and an output current. A demodulator extracts modulation from the RF signal based on a fluctuating voltage or a fluctuating current on a second coupling connection of the antenna circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/255,937, filed on Sep. 12, 2011, now U.S. Pat. No. 8,965,279, whichis a National Stage of International Application No, PCT/GB2010/050471,filed on Mar. 19, 2010, which claims the benefit of Great Britain PatentApplication No. 0904824.0, filed on Mar. 20, 2009. U.S. patentapplication Ser. No. 13/255,937 is incorporated herein by reference inits entirety.

This invention relates to near field RF communications apparatus and tonear field RF communicators and near field communications enableddevices.

Near field RF (radio frequency) communication is becoming more and morecommonplace as is the use of such technology to transfer data. Nearfield RF communicators communicate through the modulation of themagnetic field (H field) generated by a radio frequency antenna. Nearfield RF communication thus requires an antenna of one near field RFcommunicator to be present within the alternating magnetic field (Hfield) generated by the antenna of another near field RF communicator bytransmission of an RF signal (for example a 13.56 Mega Hertz signal) toenable the magnetic field (H field) of the RF signal to be inductivelycoupled between the communicators. The RF signal may be modulated toenable communication of control and/or other data. Ranges of up toseveral centimeters (generally a maximum of 1 meter) are common for nearfield RF communicators.

NFC communicators are a type of near field RF communicator that iscapable in an initiator mode of initiating a near field RF communication(through transmission or generation of an alternating magnetic field)with another near field RF communicator and is capable in a target modeof responding to initiation of a near field RF communication by anothernear field RF communicator. The term “near field RF communicator”includes not only NFC communicators but also initiator near field RFcommunicators such as RFID transceivers or readers that are capable ofinitiating a near field RF communication but not responding toinitiation of a near field RF communication by another near field RFcommunicator and target or responding near field RF communicators suchas RFID transponders or tags that are capable of responding toinitiation of a near field RF communication by another near field RFcommunicator but not of initiating a near field RF communication withanother near field RF communicator. Hence NFC communicators can act asboth RFID transceivers and RFID transponders and are able to communicatewith other NFC communicators, RFID transceivers and RFID transponders.

In addition NFC communicators may be associated with or comprised withinor attached to certain peripheral devices, for example SIM cards (e.g.UICC), Secure Elements, memory devices (for example MCU, RAM, ROM andnon-volatile memory), display driver or other drivers. During operationthe NFC communicator must also be able to communicate with and transferdata to and from such peripheral device.

There are several standards in existence which set out certaincommunication protocols and functional requirements for RFID and nearfield RF communications. Examples are ISO/IEC 14443, ISO 15693, ISO/IEC18092 and ISO/IEC 21481.

NFC communicators may be comprised within a larger device, NFCcommunications enabled devices. Examples include mobile telephones,PDAs, computers, smart cards. When comprised within such NFCcommunications enabled devices the NFC communicator must be able totransfer data to and from the larger device and to and from anyperipheral devices (including interface systems, such as the single wireprotocol) associated with such larger device.

There is pressure to reduce the space or real-estate occupied by nearfield RF communications apparatus (that is the near field RFcommunicator without the antenna circuit), particularly where the nearfield RF communicator is an NFC communicator and the near field RFcommunications apparatus is to be embedded, or otherwise incorporated,for example as an integrated circuit, in a third party host device suchas a mobile telephone (cellphone) or PDA where the overall real estateor space within the host device is already small and the space availablefor the near field RF communications apparatus is limited.

An aspect of the present invention provides near field RF communicationsapparatus that alleviates at least some of the aforementioned problems.

An aspect of the present invention provides near field RF communicationsapparatus as out in claim 1.

In an aspect there is provided a near field RF communications apparatushaving: a driver to provide an RF drive signal to drive an inductivecoupler arranged to couple via the H-field with another inductivecoupler, the RF drive signal having an RF output voltage and an RFoutput current; and a demodulator to extract modulation from an RFsignal inductively coupled to the inductive coupler of the near field RFcommunications apparatus, the driver being arranged to maintain one ofthe RF output current and the RF output voltage substantially constantsuch that variations in the antenna loading of the near field RFcommunications apparatus do not substantially vary the one of the RFoutput current and the RF output voltage to enable the demodulator toextract modulation from an RF signal on the basis of variation in theother of the RF output current and the RF output voltage.

The present invention also provides a near field RF communicator havingsuch near field communications apparatus and the inductive coupler.

Embodiments of the present invention enable the size of or real-estateoccupied by a near field RF communications apparatus to be reduced byenabling the transmit coupling connection or connections also to be usedas the receive coupling connection or connections. This reduces thetotal number of coupling connections, including transmit and receivecoupling connections, required by the near field RF communicationsapparatus. Reducing the total number of coupling connections enables thenear field RF communications apparatus to be made smaller whilst stillcomplying with minimum spacing design constraints for couplingconnections (pins in the case where the near field RF communicationsapparatus comprises an integrated circuit). Enabling the transmitcoupling connection or connections also to be used as the receivecoupling connection or connections also reduces the number of externalcomponents required where the near field RF communications apparatuscomprises an integrated circuit.

Embodiments of the present invention enable the transmit couplingconnection or connections also to be used as the receive couplingconnection or connections by maintaining one of the output voltage andoutput current supplied to the transmit coupling connection orconnections constant and detecting load modulation on the basis ofvariation of the other of the output voltage and output current suppliedto the transmit coupling connection or connections, thereby reducing thetotal number of coupling connections required by the near field RFcommunications apparatus and so reducing real-estate requirements andreducing the number of external components as discussed above.

Embodiments of the present invention provide near field communicationsapparatus comprising a driver that provides a drive signal having anoutput voltage and an output current to drive an inductive coupler,generally an antenna circuit, of a near field RF communicator such as anNFC communicator, and a demodulator that extracts modulation from an RFsignal coupled to the inductive coupler. The driver is arranged tomaintain one of the output current and the output voltage substantiallyconstant to enable the demodulator to extract modulation from an RFsignal on the basis of variation in the other of the output current andthe output voltage.

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 shows a representational diagram illustrating communicationbetween two devices comprising NFC communicators;

FIG. 2 shows a functional block diagram of a NFC communicator comprisingNFC communications apparatus and an antenna circuit;

FIG. 3 shows an example of an antenna circuit that may be used in theNFC communicator shown in FIG. 2;

FIGS. 4 and 5 illustrate examples of drivers for providing a constantoutput voltage for an antenna circuit of NFC communications apparatusembodying the invention; and

FIG. 6 illustrates an example of a driver for providing a constantoutput current for an antenna circuit of NFC communications apparatusembodying the invention.

With reference to the drawings in general, it should be understood thatany functional block diagrams are intended simply to show thefunctionality that exists within the device and should not be taken toimply that each block shown in the functional block diagram isnecessarily a discrete or separate entity. The functionality provided bya block may be discrete or may be dispersed throughout the device orthroughout a part of the device. In addition, the functionality mayincorporate, where appropriate, hard-wired elements, software elementsor firmware elements or any combination of these. The near field RFcommunicator may be provided wholly or partially as an integratedcircuit or collection(s) of integrated circuits.

Referring now specifically to FIG. 1, there is shown a representationaldiagram illustrating communication between two NFC communicationsenabled devices. In FIG. 1 the representations of the NFC communicationsenabled devices have been shown partly cut-away and the functionalityprovided by the NFC communications enabled devices illustrated by way ofa functional block diagram within the NFC communications enabled device.

As shown in FIG. 1, one NFC communications enabled device comprises amobile telephone (cellphone) 1 and the other NFC communications enableddevice comprises a portable computer 2 such as a notebook or laptopcomputer.

The mobile telephone 1 has the usual features of a mobile telephoneincluding mobile telephone functionality 10 (in the form of, usually, aprogrammed controller, generally a processor or microprocessor withassociated memory or data storage, for controlling operation of themobile telephone in combination with a SIM card), an antenna 8 forenabling connection to a mobile telecommunications network, and a userinterface 3 with a display 4, a keypad 5, a microphone 6 for receivinguser voice input and a loudspeaker 7 for outputting received audio tothe user. The mobile telephone also has a chargeable battery 11 coupledto a charging socket 12 via which a mains adapter (not shown) may beconnected to enable charging of the battery 11. The mobile telephone 1may have an alternative or additional power supply (not shown), forexample a reserve battery or emergency battery. The chargeable battery11 forms the primary power supply for the mobile telephone and NFCcommunicator 15. Given it is chargeable, it is designed to be removed atcertain times.

Similarly the portable computer 2 has the usual features of a portablecomputer including portable computer functionality 20 in the form of,usually, a processor with associated memory in the form of ROM, RAMand/or hard disk drive, one or more removable media drives such as afloppy disk drive and/or a CDROM or DVD drive, and possibly acommunications device for enabling the portable computer to connect to anetwork such as the Internet. The portable computer 2 also includes auser interface 21 including a display 22, a keyboard 23 and a pointingdevice, as shown a touchpad 24. The portable computer 2 also has achargeable battery 25 coupled to a charging socket 26 via which a mainsadapter (not shown) may be connected to enable charging of the battery25. Again the chargeable battery 25 is the primary power supply for theportable computer and NFC communicator 30.

In addition, as shown in FIG. 1, both NFC communications enabled devices1 and 2 have an NFC communicator 15 and 30. As shown, the NFCcommunicators 15 and 30 are incorporated within the larger devices and,as with the other functional blocks, may be discrete entities within thehost devices or may be provided by features dispersed throughout orintegrated within the host device or a part of the host device.

Each NFC communicator 15 and 30 comprises NFC operational components 16and 31 for, as will be described below, enabling control of the NFCfunctionality and generation, modulation and demodulation of an RFsignal. The functionality of the NFC communicator excluding the antennacircuit 102 and the power provider 104 is also referred to herein as theNFC communications apparatus.

Each NFC communicator 15 and 30 also comprises an antenna circuit 17 and32 comprising an inductor or coil in the form of an antenna 18 and 33.The antenna circuits 17 and 32 enable an alternating magnetic field (Hfield) generated by the antenna of one near field RF communicator 15 (or30) by transmission of an RF signal (for example a 13.56 Mega Hertzsignal) to be inductively coupled to the antenna of the other near fieldRF communicator 30 (or 15) when that antenna is within the near field ofthe RF signal generated by the one near field RF communicator 15 (or30).

The NFC communicators 15 and 30 are coupled to the mobile telephone andportable computer functionality 10 and 20, respectively, to enable dataand/or control commands to be sent between the NFC communicator and thehost device and to enable user input to the NFC communicator.Communication between the user interface 3 or 21 and the NFCcommunicator 15 or 30 is via the host device functionality 11 or 20,respectively,

Each NFC communicator 15 and 30 also comprises a power provider 19 and34. The power providers 19 and 34 may be power supplies within the hostdevice or specific to the NFC communicators 15 and 30, for example abutton cell battery, or other small battery. In this case as shown bydashed lines in FIG. 1, one or both of the power providers 19 and 34comprise a coupling to derive power from the corresponding devicebattery 11 or 25 i.e. the primary power supply.

It will be appreciated that FIG. 1 shows only examples of types of hostdevices. A host device may be another type of electrical device such asa personal digital assistant (PDA), other portable electrical devicesuch as a portable audio and/or video player such as an MP3 player, anIPOD®, CD player, DVD player or other electrical device. As anotherpossibility the NFC communicator (15 or 3) may be comprised within orcoupled to a peripheral device, for example in the form of a smart cardor other secure element which may be stand alone or comprised within orintended to be inserted into another electrical device. For example aSIM card for use in a mobile telephone. As a further possibility suchperipheral devices may comprise interfacing systems or protocols such asthe single wire protocol.

Also, rather than being incorporated within the host device, the NFCcommunicator 15 or 30 may be associated with the host device, forexample by a wired or wireless coupling. In such a case, a housing ofthe NFC communicator may be physically separate from or may be attachedto the housing of the host device; in the later case, the attachment maybe permanent once made or the NFC communicator may be removable. Forexample, the NFC communicator may be housed within: a housing attachableto another device; a housing portion, such as a fascia of the NFCcommunications enabled device or another device; an access card; or mayhave a housing shaped or configured to look like a smart card. Forexample an NFC communicator may be coupled to a larger device by way ofa communications link such as, for example, a USB link, or may beprovided as a card (for example a PCMCIA card or a card that looks likea smart card) which can be received in an appropriate slot of the largeror host device.

In addition, one or both of the NFC communications enabled devices maybe a standalone NFC communicator, that is it may have no functionalitybeyond its NFC communications functionality.

FIG. 2 shows a functional block diagram of an example of an NFCcommunications enabled device 100 to illustrate in greater detail oneway in which an NFC communicator may be implemented.

In this example, the NFC communications enabled device 100 comprises anNFC communicator 100 a including an antenna circuit 102, power provider104 and NFC operational components or NFC communications apparatus 100b.

The power provider 104 may be any one or more of the types of powerproviders discussed above. In the interests of simplicity, power supplycouplings from the power provider 104 to other components are not shownin FIG. 2.

The NFC communications enabled device 100 may or may not also have or becapable of being connected or coupled with at least one of otherfunctionality 105 (for example functionality of a host device orperipheral device such as described above) and a user interface 106.

The NFC operational components include a controller 107 to controlsoverall operation of the NFC communicator. The controller 107 is coupledto a data store 108 for storing data (information and/or control data)to be transmitted from and/or data received by the NFC communicationsenabled device. The controller 107 may be a microprocessor, for examplea RISC processor or other microprocessor or a state machine. Programinstructions for programming the controller and/or control data forcommunication to another near field RF communicator may be stored in aninternal memory of the controller and/or the data store.

The NFC operational components also include a demodulator 114 coupledbetween the antenna circuit 102 and the controller 107 for demodulatinga modulated RF signal inductively coupled to the antenna circuit 102from another near field RF communicator in near field range and forsupplying the thus-extracted data to the controller 107 for processing.The demodulator may be an IQ demodulator as described in WO2006/123170,the whole contents of which are hereby incorporated by reference, or anyother suitable demodulator. The received signal is also supplied to arectifier 301 and a regulator 303. The regulator 303 sets the requiredvoltage for the NFC operational components (pin voltage in the case ofan integrated circuit) and rectifier 301 provides rectified voltage toremainder of the NFC circuit. For simplicity, the couplings to supplythe regulated rectified voltage to power the NFC operational componentsare not shown in FIG. 2. Together the rectifier 301 and regulator 303protect the NFC operational components from high voltages received atantenna circuit 102. For example the regulator may limit the voltage to3.3 volts. Any standard regulator and rectification circuit can be usedfor this. The NFC operational components may also include an amplifierfor amplifying an RF signal inductively coupled to the antenna circuit102. As another possibility, the NFC operational components may bepowered solely by the power provider 104.

In addition the NFC operational components include functionality forenabling modulation of an RF signal to enable data to be communicated toanother near field RF communicator in near field range of the NFCcommunicator 100 a. As shown in FIG. 2, this functionality is providedby a controller 107 and a driver 111 coupled to the antenna circuit 102.In some examples, a separate signal generator that operates under thecontrol of the controller 107 may be provided. Modulation is in thisexample achieved by gating or switching on and off an RF signal inaccordance with the data to be communicated. The NFC communicator mayuse any appropriate modulation scheme that is in accordance with thestandards and/or protocols under which the NFC communicator operates. Asanother possibility a separate or further signal controller may beincorporated within the NFC operational components to control modulationof the signal generated by a signal generator in accordance with data orinstructions received from the controller 107.

The NFC communicator 100 a may operate in an initiator mode (that is asan initiating near field RF communicator) or a target mode (that is as aresponding near field RF communicator), dependent on the mode to whichthe NFC communicator is set. The mode may be determined by thecontroller 107 or may be determined in dependence on the nature of areceived near field RF signal. When in initiator mode, an NFCcommunicator initiates communications with any compatible respondingnear field RF communicator capable of responding to the initiating NFCcommunicator (for example an NFC communicator in target mode or an RFIDtag or transponder) that is in its near field range, while when intarget mode an NFC communicator waits for a communication from acompatible initiating near field RF communicator (for example an NFCcommunicator in initiator mode or an RFID initiator or transceiver). Asthus used, compatible means operable at the same frequency and inaccordance with the same protocols, for example in accordance with theprotocols set out in various standards such as ISO/IEC 18092, ISO/IEC21481, ISO/IEC 14443 and ISO/IEC 15693. NFC communicators commonlyoperate at or around 13.56 MHz.

When in initiator or target mode, the NFC communicator may communicatein accordance with an active or passive protocol. When using an activeprotocol the initiating NFC communicator will transmit an RF field andfollowing completion of its data communication turn off its RF field.The responding near field RF communicator (target) will then transmitits own RF field and data before again turning off the RF field and soon. When using a passive protocol the NFC communicator (initiator) willtransmit and maintain its RF field throughout the entire communicationsequence. The protocol used will depend on instructions received fromthe controller 107 and the response received from a responding nearfield RF communicator.

In FIG. 2 control of operation of the NFC communicator is throughcontroller 107. As another possibility where the NFC communicator iscomprised as part of a host device, control of the operation of the NFCcommunicator may be directed by the host device, for example throughother functionality 105. In such circumstances all or part of thecontrol may be provided by the other functionality 105. For example theNFC communicator controller 107 may control modulation and modulationprotocols whereas the data to be transmitted may be provided by theother functionality 105.

The NFC communicator also comprises an antenna circuit 102. The designof the antenna circuit will depend on the NFC communicator 100 and theenvironment in which it operates. The antenna circuit may be asingle-ended circuit with one transmit output and one receive input or adouble-ended circuit having two transmit outputs and two receive inputs,or may be single-ended for one of receive and transmit and double-endedfor the other. FIG. 2 illustrates a double-ended antenna circuit withtwo transmit outputs Tx2 and Tx2 and two receive inputs Rx2 and Rx2,Generally the functionality except the antenna circuit 102 and the powerprovider 104 may be provided by one or more integrated circuits and sothe transmit outputs Tx2 and Tx2 and receive inputs Rx1 and Rx2 will beprovided by pins of the integrated circuit. As an example the antennacircuit may be in one of the forms described for co-pendinginternational patent application publication number WO2008/117029 (NFCAntenna).

FIG. 3 shows an example of a double-ended antenna circuit that may beused for the NFC communicator 100 shown in FIG. 2. In this example, theantenna circuit comprises an antenna coil L1 (resistor R3 represents theparasitic resistance of the antenna coil L1) with one end of the antennacoil L1 being coupled to transmit pin Tx1 via a resistor R1 and acapacitor C3 and to receive pin Rx1 via a capacitor C5 and the other endof the antenna coil L1 being coupled via a resistor R2 and a capacitorC4 to transmit pin Tx2 and to receive pin Rx2 via a capacitor C6. In theexample shown, capacitors C1 and C2 are coupled in series across thetransmit pins Tx1 and Tx2. The dashed line X in FIG. 3 illustrates theperiphery of the NFC communications apparatus, that it the periphery ofthe integrated circuit in this example. It will be appreciated that FIG.3 is only an example and that many different forms of antenna circuitare possible.

Another near field RF communicator 50 (shown for simplicity in FIG. 3simply as an antenna coil L1 (with its parasitic resistor R4)) havingits antenna coil in inductive coupling (H field) range communicates datato the NFC communicator 100 by load modulation that is by causing theload on the antenna circuit of the NFC communicator 100 to vary inaccordance with the data to be communicated, thereby effecting amplitudemodulation.

The NFC communicator illustrated by FIGS. 2 and 3 requires four couplingconnections (pins in the case of an integrated circuit) Tx1, Tx2, Rx1and Rx2 for the antenna circuit while the NFC communicator as a wholemay only require 10 or so coupling connections.

There is a desire to reduce the space or real-estate occupied by an NFCcommunicator. The real estate required is generally related to number ofthe coupling connections (pins in the case of an integrated circuit) ofthe near field communications apparatus because design constraintsdetermine a minimum spacing between coupling connections. The pressureto reduce the space or real-estate occupied by near field communicationsapparatus is particularly high where the near field communicationsapparatus is to be embedded, for example as an integrated circuit, in athird party host device such as a mobile telephone (cellphone) or PDAwhere the overall real estate or space within the host device is alreadysmall and the space available for the near field communicationsapparatus is limited.

Embodiments of the present invention enable the size of the near fieldcommunications apparatus to be reduced by reducing the number ofcoupling connections (pins in the case of an integrated circuit) byenabling the transmit coupling connection or connections also to be usedas the receive coupling connection or connections.

Embodiments of the present invention enable also enable a reduction inthe number of external components required where the near field RFcommunications apparatus comprises an integrated circuit.

Embodiments of the present invention enable the transmit couplingconnection or connections also to be used as the receive couplingconnection or connections by maintaining one of the voltage and currentsupplied to the transmit coupling connection or connections constant anddetecting modulation using the other of the voltage and current suppliedto the transmit coupling connection or connections. The output currentI_(out) of the antenna circuit is proportional to the ratio of theoutput voltage V_(out) of the antenna circuit divided by an effectiveresistance which consists of the output resistance R_(out) of theantenna circuit and the load resistance R_(load′) provided by the load(the antenna circuit of another near field RF communicator, for examplea tag):

$I_{out}\alpha\;{\frac{V_{out}}{R_{out} + R_{{load}^{\prime}}}.}$

so that when the output current I_(out) is maintained constant, any loadmodulation will affect the output voltage V_(out) and so will bedetectable from variation of the output voltage V_(out), whereas whenthe output voltage V_(out) is maintained constant any load modulationwill affect the output current I_(out) and so will be detectable fromvariation of the output current I_(out).

FIGS. 4 to 6 illustrate examples of drivers that may be used in place ofthe driver 111 shown in FIG. 2 where the antenna circuit is asingle-ended antenna circuit. The drivers shown in FIGS. 4 and 5 arearranged to provide a constant output voltage V_(out) for the antennacircuit to enable load modulation to be detected on the basis ofvariation of the output current I_(out) so that only a singletransmit/receive coupling connection Tx/Rx is required. FIG. 6illustrates an example of a driver that is arranged to provide aconstant output current I_(out) for the antenna circuit to enable loadmodulation to be detected on the basis of variation of the outputvoltage V_(out). In FIGS. 4 to 6, Vdd represents the power supply line.

In the example shown in FIG. 4, the driver 111 is replaced by adifferential amplifier, in this example a class AB amplifier, 401(although any appropriate form of linear analogue driver may be used)having its positive input coupled to an oscillating signal supply outputof the controller 107 (or signal generator if a separate signalgenerator is provided). The output of the differential amplifier 401 iscoupled to its negative input via a feedback path 402 and is alsocoupled to a single transmit coupling connection or output Tx. In FIG.4, resistor R6 represents the loading of the antenna. The demodulator114 a is in this example a single input demodulator and is coupled to afeedback node Ni of the feedback path. Any suitable demodulator that iscapable of decoding changes in both amplitude and phase resulting fromload modulation may be used. In operation, the voltage at the singleoutput Tx is controlled when the NFC communicator is in target mode sothat a constant voltage V_(out) is provided at the single output Txwhilst the current through the feedback path 402 and thus at thefeedback node N1 varies in accordance with any load modulation enablingthe demodulator 114 a to extract the modulation from the varyingcurrent.

In the example shown in FIG. 5, the driver 111 is replaced by a singleinput amplifier 403 (which may be of the same type as the amplifier 401shown in FIG. 4) having its input coupled to an oscillating signalsupply output of the controller 107 (or signal generator if a separatesignal generator is provided). The output of the amplifier 403 iscoupled to a single transmit coupling connection or output Tx. ResistorR7 represents the loading of the antenna. A sense resistor R8 isprovided in the amplifier power supply coupling to Vdd and anappropriate single input demodulator 114 b is coupled to a node N2.Again any suitable demodulator that is capable of decoding changes inboth amplitude and phase resulting from load modulation may be used. Inoperation, the voltage at the single output Tx is controlled when theNFC communicator is in target mode so that a constant voltage V_(out) isprovided whilst the current through the sense resistor R8 varies inaccordance with any load modulation enabling the single inputdemodulator 114 b to extract modulation on the basis of variation of thecurrent flowing through the sense resistor R.

in the example shown in FIG. 6, the controller 107 is arranged toprovide an oscillating signal, for example by sine synthesis, to twovariable current sources 500 and 501 through which the currents areadjusted or controlled when the NFC communicator is in target mode sothat the current at a junction J1 between the two variable currentsources 500 and 501 remains constant. The junction J1 is coupled to asingle transmit coupling connection or output Tx, In this Figure, theresistor R9 represents the loading of the antenna. The transmit couplingconnection Tx is coupled to the demodulator 114 c. In operation, thecurrent at the single output Tx is controlled when the NFC communicatoris in target mode so that a constant current I_(out) is provided whilstthe output voltage varies in accordance with any load modulation,enabling the single input demodulator 114 c to extract modulation on thebasis of variation of the output voltage.

It will be appreciated that other drivers than those described above maybe used to enable either the output current or the output voltage to beheld constant. It will of course be appreciated that in an NFCcommunicator the output current and output voltage oscillate. Wherereference is made herein to keeping or maintaining a voltage output or acurrent output constant, it should be understood that what is meant isthat the loading on the antenna does not affect it, that is the voltageor current, as the case may be, does not change with the output loadresistance. It may be in practice that some variation with loadresistance may be acceptable in the constant voltage or constantcurrent, provided that the modulation is still detectable by thedemodulator.

The term “load” is used herein to mean antenna loading, i.e. theelectrical impedance of an antenna (or inductive coupler) due toinductive loads (impedances) which are inductively coupled to it. Thedrive signal may be any oscillatory signal and need not be radiofrequency, RF. The actual frequency of this oscillatory signal maydepend, for example, upon the parameters (tuning) of the inductivecouplers

Although single-ended interfaces and circuits are shown in the examplesof FIGS. 4 to 6, it will be appreciated that the invention may beapplied where differential interfaces and circuits are used and alsowhere a combination of single-ended and differential interfaces andcircuits are used.

It will be appreciated that the above description is directed to theoperation of the NFC communicator in target, mode and that, when the NFCcommunicator is in initiator mode, then, in order to allow modulation,the controller will modulate the constant current or voltage, as thecase may be, in accordance with the data to be transmitted. It will beappreciated that in the initiator mode when the antenna driver anddemodulator are turned on, the rectifier and regulator (FIG. 2) areturned off and in the target mode when the antenna driver, anddemodulator are turned off, the rectifier and regulator (FIG. 2) areturned on, so that the rectifier and regulator do not interfere with theantenna driver. Thus, in the target mode the regulator and rectifierwill behave benignly, that is as though they were not present.

In the above embodiments the invention is described in connection withan NFC communicator. It will be apparent to the skilled man that thesame system could be used in any near field RF communicator capable ofdetecting load modulation and has particular advantages where space is apremium, for example in a host device or where the housing or package ofa stand-alone near field RF communicator has to be small because of theapplication for which it is to be used.

The above embodiments are to be understood as illustrative examples ofthe invention. Further embodiments of the invention are envisaged. It isto be understood that any feature described in relation to any oneembodiment may be used alone, or in combination with other featuresdescribed, and may also be used in combination with one or more featuresof any other of the embodiments, or any combination of any other of theembodiments. Furthermore, equivalents and modifications not describedabove may also be employed without departing from the scope of theinvention, which is defined in the accompanying claims.

The invention claimed is:
 1. A communication device, comprising: acoupling connection configured to receive a communication signal from asecond communication device, the communication signal causing an outputcurrent at the coupling connection to fluctuate; a driver configured toprovide a radio frequency (RF) drive signal, having the output currentand an output voltage, to the coupling connection and to adjust theoutput current to substantially compensate for fluctuations in theoutput current to maintain the output voltage at a substantiallyconstant voltage at the coupling connection; a feedback path, coupledbetween an output of the driver and an input of the driver, configuredto provide a feedback current between the output of the driver and theinput of the driver; and a demodulator, coupled to a feedback node ofthe feedback path, configured to extract modulation from the feedbackcurrent, the feedback current fluctuating in a substantially similarmanner as the output current.
 2. The communication device of claim 1,wherein the communication device comprises: a near field communication(NFC) device configured to operate in a target mode of operation.
 3. Thecommunication device of claim 1, wherein the input of the drivercomprises: a negative input; and a positive input, and wherein thefeedback path is coupled between the output of the driver and thenegative input of the driver.
 4. The communication device of claim 1,wherein the driver is further configured to maintain the output voltageat the substantially constant voltage so that the fluctuations of theoutput current do not substantially vary the output voltage.
 5. Thecommunication device of claim 1, wherein the demodulator is furtherconfigured to detect the modulation based on the fluctuations of thefeedback current as the driver maintains the output voltage at thesubstantially constant voltage.
 6. The communication device of claim 1,further comprising: an antenna circuit configured to receive thecommunication signal from the second communication device, and whereinthe second communication device is configured to cause a load on theantenna circuit to vary in accordance with the communication signal. 7.The communication device of claim 6, wherein the demodulator is furtherconfigured to detect the modulation based on the fluctuations of theoutput current at the load of the antenna circuit.
 8. The communicationdevice of claim 1, wherein the demodulator is further configured todecode a change in amplitude or in phase of the feedback currentresulting from the fluctuations of the feedback current.
 9. A method,comprising: causing an output current of a communication device tofluctuate in response to a communication signal received a secondcommunication device; providing a radio frequency (RF) drive signal,having the output current and an output voltage; adjusting the outputcurrent to substantially compensate for fluctuations in the outputcurrent to maintain the output voltage at a substantially constantvoltage; feeding back the output current in the communication device toprovide a feedback current that fluctuates in a substantially similarmanner as the output current, extracting modulation from the feedbackcurrent; and controlling the communication device to maintain the outputvoltage at the substantially constant voltage.
 10. The method of claim9, wherein the controlling comprises: maintaining the output voltage atthe substantially constant voltage so that the fluctuations of theoutput current do not substantially vary the output voltage.
 11. Themethod of claim 9, further comprising: detecting the modulation based onthe fluctuations of the feedback current as a driver maintains theoutput voltage at the substantially constant voltage.
 12. The method ofclaim 11, wherein the detecting comprises: detecting the modulationbased on the fluctuations of the output current.
 13. The method of claim9, further comprising: decoding a change in amplitude or in phase of thefeedback current resulting from the fluctuations of the feedbackcurrent.
 14. A communication device, comprising: a driver configured toprovide a radio frequency (RF) drive signal, having the output currentand an output voltage, and to adjust the output current to substantiallycompensate for fluctuations in the output current caused by acommunication signal to maintain the output voltage at a substantiallyconstant voltage at an output of the driver; a feedback path, coupledbetween the output of the driver and an input of the driver, configuredto provide a feedback current between the output of the driver and theinput of the driver; and a demodulator, coupled to a feedback node ofthe feedback path, configured to extract modulation from the feedbackcurrent, the feedback current fluctuating in a substantially similarmanner as the output current.
 15. The communication device of claim 14,further comprising: a coupling connection configured to receive thecommunication signal from a second communication device.
 16. Thecommunication device of claim 14, further comprising: a controllerconfigured to control the driver to maintain the output voltage at thesubstantially constant voltage.
 17. The communication device of claim14, wherein the communication device comprises: a near fieldcommunication (NFC) device configured to operate in a target mode ofoperation.
 18. The communication device of claim 14, wherein the driveris further configured to maintain the output voltage at thesubstantially constant voltage so that the fluctuations of the outputcurrent do not substantially vary the output voltage.
 19. Thecommunication device of claim 14, wherein the demodulator is furtherconfigured to detect the modulation based on the fluctuations of thefeedback current as the driver maintains the output voltage at thesubstantially constant voltage.
 20. The communication device of claim19, wherein the demodulator is further configured to detect themodulation based on the fluctuations of the output current at a load ofan antenna circuit.
 21. The communication device of claim 14, whereinthe demodulator is further configured to decode a change in amplitude orin phase of the feedback current resulting from the fluctuations of thefeedback current.